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  1. #1
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581

    Finally got static pressure down to ok level

    My system is a Trane split XV95-100btu/XL15i-4ton.

    For historical reading you may go to the following link.

    http://hvac-talk.com/vbb/showthread.php?t=177106&page=4

    But here is a summary.

    My system started with a total external static pressure of ~1.1 (on cool) with 3 trunks coming off the supply plenum. A 12x9, a 12x8, and a 8x9.5 all leading to a total of 16 supply runs where three were 6" round and 13 were 5" round...all metal ducts (no flex).

    I am happy to say I am now at 0.75 tesp (down from 1.1) and the upstairs is within a degree or 2 of the downstairs (was 12 degrees hotter).....without tearing out any drywall and leaving my small 5 or 6 inch wall stacks in place.

    I realize now after the fact that the problem was my system was likely oversized.

    Here is the list of things that they did to ultimately resolve the problem.
    I'm sure some contributed more than others.

    1. Converted the 12x8 trunk off the plenum to a 14x14.
    2. Converted the 8x9.5 trunk off the plenum to a 16x8.
    3. Changed the return drop from a 24x10 to a 24x20.
    4. Added a 5" media filter 25x20 horizontial in the return drop to replace the 1" restrictive electrostatic OEM filter located right at the blower.
    5. opened all the registers and removed the "restrictor" plates that were designed to increase "throw" by adding static pressure.
    6. Increased 2 upstairs return grills from 10x6 to 14x14.
    7. Sealed in between the drywall and the register/return grills with mastic or foil tape.
    8. Sealed the return and supply leaks in the basement around the furnace/coil as well as in the ductwork.
    9. Added R-30 insulation batts to the attic.
    10. Cut the Bk jumper on the furnace board to reduce airflow to 80%. This effectively makes the system a 3.2 ton instead of a 4-ton....delivering 1280 CFMs instead of 1600. The ironic part is I never got 1600 CFM anyway because the static was so high I got closer to 1250-1300 CFM.
    ~1300 CFM cools my house very well.

    According to the response I got from Trane, the system can be run up to static pressures of 0.9 (with a reduction in CFM due to the static).

    For heating, the dip switches were just placed on the lowest fan-speed where stage 1 -is 900CFM and stage 2 is ~1300CFM.

    Thanks to all on this site who contributed to the resolution.

    I'd like to add some special thanks to ......well you all know who you are.

    Key1
    In the land of the blind.....the man with one eye is king....

  2. #2
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989

    Congratulations to everyone involved - Great Work

    It has been great to see your results.
    That sure is a good feeling of accomplishing something worthwhile, huh.
    Congratulations to everyone involved!
    That's what it's all about. - Darrell

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,081
    Congrats.
    You'll get a lot more life span from your air filter now, then you would have before.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    I forgot to mention an 8" round duct off the plenum was also added, and that the old "eL" in the return drop had a square front and a radius back while the new larger one has radius curvature in the front and back.

    I know there is more that can be done to get the TESP even lower such as tearing out the drywall and upsizing all the wall stacks and register boots..... or by adding turning vanes at key restrictive turns ....but for now...I'm happy.

    By the way we could not find turning vanes. I went with my duct guy to the local supply house to show pictures of vanes I got from this site and they looked at us like a deer in the headlites. They said they never sold any of them and no one has ever asked for them .

    This is a big chain HVAC/Plumbing Supply house. They did indicate that they could order some but it would have to be in 100 ft quantities (way too much) and it would take several weeks....we passed on that.

    Key1
    In the land of the blind.....the man with one eye is king....

  5. #5
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by beenthere View Post
    Congrats.
    You'll get a lot more life span from your air filter now, then you would have before.
    The pressure drop across my Merv 10 media 5" filter is now only 0.10 (filter been in use for ~ 3 weeks).
    The pressure drop acoss the coil is down from 0.4 to 0.3 at the new reduced airflow.

    The calculated velocity through the filter is ~369 FPM from:
    20x25 = 500 sq. inches filter area
    500/144 = 3.4722 sq. ft.

    1280 CFM = 1280 cubic ft/min

    1280/3.4722 = 369 feet per minute



    Key1
    In the land of the blind.....the man with one eye is king....

  6. #6
    Quote Originally Posted by key1cc View Post
    My system is a Trane split XV95-100btu/XL15i-4ton.
    ...

    10. Cut the Bk jumper on the furnace board to reduce airflow to 80%. This effectively makes the system a 3.2 ton instead of a 4-ton....delivering 1280 CFMs instead of 1600. The ironic part is I never got 1600 CFM anyway because the static was so high I got closer to 1250-1300 CFM.
    ~1300 CFM cools my house very well.
    Key1,

    I've been following your static pressure saga with great interest, given that I have been seeing concerns in my new system (almost 0.95 total static, and 0.40-0.45 across the coil) - so I am curious about some of your observations/methods.

    First, did you conclude that your coil is a 4 ton or 5 ton unit? I remember that beenthere said that the model you provided referred to a 5 ton coil - did you conclusively determine that it's a 4 ton?

    The reason I ask is that I am curious about your flow rates - wouldn't 1300 cfm be way too low for a 5 ton? And even for a 4 ton, that would only give you about 325 cfm/ton - did your contractor have any concerns with that?

    My 5 ton system was initially set up for 400 cfm/ton (2000 cfm), but that resulted in 1.0 ESP, so he dropped it to 360 cfm/ton, or about 1800 cfm - that lowered it to 0.9 ESP. I'm wondering if he should try to lower it even more, to say 1600 cfm, to drop the ESP further?

    Would be curious about your thoughts, given all the efforts you have gone through, and all the data you collected!

    I do have to say that I am really happy that I got a 2 stage heat/cool system, as most of the time it runs on 1st stage, where the ESP is around 0.4, phew!

    Thanks,

    Mark

  7. #7
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by mark johnson View Post
    Key1,

    I've been following your static pressure saga with great interest, given that I have been seeing concerns in my new system (almost 0.95 total static, and 0.40-0.45 across the coil) - so I am curious about some of your observations/methods.

    First, did you conclude that your coil is a 4 ton or 5 ton unit? I remember that beenthere said that the model you provided referred to a 5 ton coil - did you conclusively determine that it's a 4 ton?

    The reason I ask is that I am curious about your flow rates - wouldn't 1300 cfm be way too low for a 5 ton? And even for a 4 ton, that would only give you about 325 cfm/ton - did your contractor have any concerns with that?

    My 5 ton system was initially set up for 400 cfm/ton (2000 cfm), but that resulted in 1.0 ESP, so he dropped it to 360 cfm/ton, or about 1800 cfm - that lowered it to 0.9 ESP. I'm wondering if he should try to lower it even more, to say 1600 cfm, to drop the ESP further?

    Would be curious about your thoughts, given all the efforts you have gone through, and all the data you collected!

    I do have to say that I am really happy that I got a 2 stage heat/cool system, as most of the time it runs on 1st stage, where the ESP is around 0.4, phew!

    Thanks,

    Mark
    My indoor coil, also known as A-coil, also known as e-coil for evap coil is 5 ton as beenthere pointed out. My outside unit is 4-ton. I know now this configuration was done for efficiency to get me up to 14.5 seer.

    When I think about CFMs I usually think of it compared to the outside coil not the inside one. Thus the rule of thumb of 400 CFM/ton would suggest that the optimum airflow for my 4-ton unit is 1600 CFM. I came to the conclusion that when the comfort-R program is set on my system, then the 1st 8.5 minute of every cycle is at 80% airflow or less. If my cycles per hour are set at 3, then my unit is on for about 10 minutes and off for about 10 minutes 3 times an hour (roughly). That means that 85% of the time that my condenser is on, the air flow is at 80% of 1600 CFM or 1280CFM!
    This is not my configuration but is how the engineers designed the unit to operate.

    So I surmised that if it is designed so that it can run at 80% air flow 85% of the time......then I may be ok running it at 80% of air flow all of the time.

    So I have been and am still running at 80% airflow 100% percent of the time and I have 4 of 17 dampers closed so that the house cools nice and evenly on both floors. The static under these conditions is at 0.75 even with a 5" merv 10 media filter and I am very happy with the performance. One possible downside that I am keeping a lookout for is potential coil freezing that so far does not appear to be an issue. 1280 CFM is 320 CFM/Ton which I beleive Beenthere said is where he runs his system. Heck, the Honeywell website talks about built in controls on there Tstat not allowing you to go below 280 CFM/ton!

    Since cutting the BK jumper had no impact on heating...... the reduction of the airflow in the heating mode was done with the dip switches and the unit was set to run in a permenant stage 1 by deceiving the Tstat and telling it that I have a "1 Cool 1 Heat" unit instead of a "2-heat" unit. The medium setting delivers ~ 1200 CFMs at a total ESP of 0.67.

    What did my contractor say you ask? I ended up dealing with the owner and after he finished the "deer in the headlight look" he asked me did I want a job . He basically followed the instructions I gave him which I got from some really good techs on this site. I suspect he was so helpful because I never reported him to Trane for leaving me with an install at 1.1 static pressure.

    The reality is I bought more system than I needed because I could be getting the same performance with a 3.2 ton condenser and 63K btu single stage furnace.......Like you, I too am glad I have the 2-stage and the variable to give me flexibility to compensate for the oversizing.

    Key1
    In the land of the blind.....the man with one eye is king....

  8. #8
    Thanks Key1, I am learning a lot from your experiences! So it sounds like 320 cfm/ton is not too extreme, which might allow me to run my 5 ton system at 1600 cfm instead of the current 1800 cfm... I suspect that would lower the static to about .75-.80, not too bad.

    How are you keeping an eye out for coil freezing, BTW? Pop open the cover, or are there other tell-tale signs?

    What I do not understand for the life of me (which I think you can relate to from your experiences) is this - my 5 ton e-coil (which is supposedly designed for 2000 cfm flow) has a PD across the coil of 0.50" at 2000 cfm, and 0.40" at 1800 cfm - and yet the coil instructions say to maintain PD across coil of 0.30" or less! Seems to me that the manufacturer would have tested this coil and seen that it cannot possibly achieve 2000 cfm with 0.30" PD - it would probably need to be closer to 1500-1600 cfm, from what I observed, which is not in line with their recommended 400 cfm/ton.

    Mark

  9. #9
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by mark johnson View Post
    Thanks Key1, I am learning a lot from your experiences! So it sounds like 320 cfm/ton is not too extreme, which might allow me to run my 5 ton system at 1600 cfm instead of the current 1800 cfm... I suspect that would lower the static to about .75-.80, not too bad.

    How are you keeping an eye out for coil freezing, BTW? Pop open the cover, or are there other tell-tale signs?

    What I do not understand for the life of me (which I think you can relate to from your experiences) is this - my 5 ton e-coil (which is supposedly designed for 2000 cfm flow) has a PD across the coil of 0.50" at 2000 cfm, and 0.40" at 1800 cfm - and yet the coil instructions say to maintain PD across coil of 0.30" or less! Seems to me that the manufacturer would have tested this coil and seen that it cannot possibly achieve 2000 cfm with 0.30" PD - it would probably need to be closer to 1500-1600 cfm, from what I observed, which is not in line with their recommended 400 cfm/ton.

    Mark
    You have to ask yourself a couple of questions.
    1. Will the reduced CFM's still cool/heat my home to my satisfaction?
    2. Will the reduced airflow negatively effect my equipment?

    If yes to 1 and no to 2 you are in good shape with the caveats that you as a HO (Home owner) can easily tell if 1 is happening.....

    I've learned from this site that one of the things that can cause coil freezing is restricted airflow (like from clogged filters etc.). I'm not really concerned about it with controlled reduction of airflow because as I pointed out, the Engineers that designed the system designed it to flow at 80% on the comfort-R setting (also known as the enhanced mode). The comfort R setting instructions indicate that the system will run at 50% airflow the 1st 1 minute then 80% the next 7.5 minutes and then 100% IF NEEDED. That "if needed" tells me that if not needed it will cycle off and continue to run up to 80% when it cycles back on.

    I suspect that with a really clogged filter (not being changed for over a year) the reduction in airflow would be far more than 80%. Maybe some pro's that encountered this and can provide us some insight.

    Calling all pros's.....Should we be concerned about coil freezing when running at 80% airflow?

    Key1
    In the land of the blind.....the man with one eye is king....

  10. #10
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,989
    Quote Originally Posted by mark johnson View Post
    Thanks Key1, I am learning a lot from your experiences! So it sounds like 320 cfm/ton is not too extreme, which might allow me to run my 5 ton system at 1600 cfm instead of the current 1800 cfm... I suspect that would lower the static to about .75-.80, not too bad.

    How are you keeping an eye out for coil freezing, BTW? Pop open the cover, or are there other tell-tale signs?

    What I do not understand for the life of me (which I think you can relate to from your experiences) is this - my 5 ton e-coil (which is supposedly designed for 2000 cfm flow) has a PD across the coil of 0.50" at 2000 cfm, and 0.40" at 1800 cfm - and yet the coil instructions say to maintain PD across coil of 0.30" or less! Seems to me that the manufacturer would have tested this coil and seen that it cannot possibly achieve 2000 cfm with 0.30" PD - it would probably need to be closer to 1500-1600 cfm, from what I observed, which is not in line with their recommended 400 cfm/ton.
    Mark
    The mfg'ers are using the coil Pressure Drop (PD) to target the delivered CFM through the coil.

    They should have listed the resultant CFM at a 0.30" PD.

    The use of more tonnage combined with a lower airflow lowers the coil temperature & reduces the nominal BTUH at those load conditions. That results in longer run-times which increases SEER performance & also helps in the handling of the humidity latent load.

    I could say a lot more about the design engineering of the high SEER units, but would rather not expound on those issues in this forum.

    With light load conditions & lower outdoor temps, freeze-up of the Coil could be a problem, in many climate conditions I would want a freeze stat on it.

    I do know that in some installations the large physical size of these new coils create problems. That is one of many reasons - to invest more money & attention to reducing the heat-gain toward reducing the A/C tonnage requirements, allowing proper sizing for longer run-time efficiencies! - Darrell

  11. #11
    Quote Originally Posted by key1cc View Post
    1. Will the reduced CFM's still cool/heat my home to my satisfaction?
    In the mild climate where we live (So CA), my system runs on first stage almost all the time, and the second stage is there as "backup" for the more extreme days, so I believe the cooling/heating ability will still be there.

    2. Will the reduced airflow negatively effect my equipment?
    Yes, this is the concern at hand. I am not worried about the heating part (we had the furnace set to the absolute lowest speed last winter, and temp rise was well within spec), but with the AC now added, I want to make sure that there is sufficient airflow across the evap.

    I'm not really concerned about it with controlled reduction of airflow because as I pointed out, the Engineers that designed the system designed it to flow at 80% on the comfort-R setting (also known as the enhanced mode). The comfort R setting instructions indicate that the system will run at 50% airflow the 1st 1 minute then 80% the next 7.5 minutes and then 100% IF NEEDED.
    OK, maybe this is my HO ignorance showing, but it would seem to me that the "80%" value is only meaningful if referring to a nominal value that is known to be "adequate". In other words, if it is known that the 1600 cfm airflow is "adequate" (i.e. adequate to keep the coil from freezing etc), then 80% of that (1280 cfm) may still be OK. But, how did you determine that the 1600 cfm was "adequate" to begin with? With the 5 ton evap coil, that's only 320 cfm/ton. You mentioned that beenthere stated that 320 is a reasonable number - but is it reasonable to drop it yet an additional 20% like you did? With the 1300 cfm you now have across the evap, that's only 260 cfm/ton!

    Don't get me wrong, I am not trying to punch holes in your approach, I would actually like to duplicate what you did so that I too can lower my static - I am just still trying to catch up with the logic .

    Thanks key1!

    Mark

  12. #12
    Quote Originally Posted by udarrell View Post
    The mfg'ers are using the coil Pressure Drop (PD) to target the delivered CFM through the coil.

    They should have listed the resultant CFM at a 0.30" PD.
    In my case, they only state "adjust airflow to achieve 0.30" max", which I expect to be around 1500 cfm - a bit low for a 5 ton coil, it seems.

    With light load conditions & lower outdoor temps, freeze-up of the Coil could be a problem, in many climate conditions I would want a freeze stat on it.
    And that is my concern, living in mild temp/low humidity conditions!

  13. #13
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by mark johnson View Post
    ....... But, how did you determine that the 1600 cfm was "adequate" to begin with.....
    Mark

    my installation manual indicates to not use comfort r on the 350/ton setting. it states to only use it on the 400cfm/ton or higher setting......which is 1600cfm......that is how i determined it was adequate...

    Now let me help you out with your concerns on the 5-ton evap coil because i don't think your current thinking is accurate. First of all the outside unit which is the 4-ton condenser is what is generating the cooling btu's. The cooling btu's are not being generated by the inside coil.
    the inside coil could have just as easily been 4-ton.....but 5-ton was used for greater efficiency...but still only 4 tons worth of cooling is going through it because the cooling btu's are generated by the outside unit......soooooo it is that 4 tons of cooling that we need to protect from too low air flow not 5 tons).

    If a pro wants to jump in and confim or deny this ....please feel free.

    Key1
    In the land of the blind.....the man with one eye is king....

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